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Septic arthritis in foals

During foaling season, equine practitioners are asked to examine foals that present with lameness or joint effusion. Many times the owners will report that the foal was noticed to be a little "off" for the past few days, and they assumed the mare stepped on it. These words should alert the practitioner to the real possibility of the foal having a septic arthritis or osteomyelitis. Because of the seriousness of the potential problem, all lame neonatal foals should be considered to have a septic joint, epiphysis or physis until proven otherwise.

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Septic arthritis in the foal often is accompanied by a history of some incident that may have resulted in a decrease in the ingestion of adequate maternal antibodies from colostrum. The mare might have leaked colostrums before parturition. The foal might have been delayed in suckling. One study by McCoy found that the foals with failure of passive transfer were 1.7 to 1.9 times more likely to develop septic arthritis or septic osteomyelitis, respectively, than foals that received adequate colostrum.

Foals presenting with septic arthritis are usually less than 1 month of age. Clinically, the younger (birth to 36 hours) neonatal foal presents with joint effusion as a part of general signs of septicemia in the first day of life. Alternatively, the slightly older neonate (>36 hours) may appear relatively normal at first and develop lameness over a period of days or even weeks. Owners might report that the foal seems to spend more time recumbent than other foals. Evidence of thickened or leather-like skin over boney prominences, such as the elbows, hips, hocks and carpi, are the signs of the development of pressure decubital ulcers secondary to orthopedic pain and recumbency (Photo 1).

Careful palpation of all joints and physis should be done on the physical examination of the neonatal foal. Though all joints and growth plates in the foal have the potential for hematogenous infection, the joints of the axial skeleton are affected most often. The site prevalence varies with different studies, but the tarsus, stifle, fetlock and carpus are affected frequently. Subtle joint effusion can be detected by palpating the opposite joint for comparison. Septic physitis may not present with joint swelling but rather with edema over the growth-plate region. Pressure at this site can illicit a painful response from the foal.

Arthrocentesis of a distended joint is usually straightforward. The joint to be aspirated is surgically prepared and an 18-gauge or 20-gauge needle is directed into the distended joint capsule. Landmarks for the different joints are published. The clinician should aspirate enough fluid to place it in an EDTA tube for fluid analysis, blood culture media for incubation and on a culturette for immediate plating.

Normal synovial fluid analysis yields a cell count of less than 250 cells/mm3. Though the differential count should have a mixture of neutrophils, lymphocytes and mononuclear cells, neutrophils should make up less than 10 percent of the distribution. Protein levels in the fluid can be measured on a refractometer and should be less than 2 gm/dl. Normal synovial fluid has a high level of viscosity. This can be determined practically by watching a fluid drop from the syringe or placing a drop of fluid between two fingers. Normal synovial fluid should have a stringiness to it.

Synovial fluid from infected joints has a high WBC count and an elevated protein. Cell counts can range from several thousand to several-hundred thousand. The differential of cells in the infected joint is generally >90 percent neutrophils. Bacteria occasionally can be seen on a Gram stain of the fluid. The joint fluid from infected joints becomes serous in nature, losing its stringiness.

Imaging
Radiographic examination of effected joints is helpful in determining the primary site of the joint that is infected, but often bone involvement is not seen in the early stages of the disease process. Computerized tomography (CT) has been shown to demonstrate osteomyelitis in foals before radiographic changes; therefore, it might be helpful in identifying boney involvement at an earlier stage of the disease process (Photo 1). If CT is not available, and the foal does not respond to therapy, then radiographs should be repeated every three to five days. Epiphyseal or physeal lesions may become more evident over time.